1. Technical Field
The present invention relates to a method and apparatus for inspecting a pattern formed on a substrate having a fine circuit pattern, such as a semiconductor product or liquid crystals, using an electron beam.
2. Background Art
A semiconductor wafer inspection apparatus will be described by way of example. Semiconductor products are manufactured by repeating the lithography and etching processes whereby a pattern formed in a photomask placed on a semiconductor wafer is transferred. During the manufacture of semiconductor devices, the quality of the pattern formed by the lithography and etching processes and the presence of foreign matter, for example, greatly affects the yield of the semiconductor devices. Accordingly, it is necessary to detect the presence of abnormality, particles or defects at an early stage or even before they occur.
At the initial or developing stages of a manufacturing process involving a semiconductor device, various kinds of defects are present in large amounts, each kind of defect having its own cause and therefore requiring different countermeasures or different locations for providing the countermeasures. It is difficult to deal with all of the defects at once, with some defects sometimes affecting one another. Thus, such countermeasures can never be complete in a single operation. If a decrease in a certain defect is observed after a plurality of countermeasures has been implemented, it can be concluded that these countermeasures have been effective. However, when a large number of defects of various kinds are present at an early stage, it is extremely difficult to find out which defect among many can be reduced by which countermeasure. Manufacturing process developers devote their knowledge and effort to finding the optimum process conditions by scrutinizing the defects, the outcome of which depends to a great extent on the experience of experts.
Apparatuses for inspecting defects existing in the pattern on a semiconductor wafer include an optical inspection apparatus that irradiates the semiconductor wafer with white light and compares similar circuit patterns of LSIs using optical images. Another example is an electron beam inspection apparatus, whereby secondary or reflected electrons generated by electron beam irradiation are detected to produce an image such as an SEM, which is then compared with a reference image to detect defects or foreign matter. The term SEM stands for scanning electron beam microscopy.
In these inspection apparatuses, an inspection image and a reference image are compared, and only differing pixels in binarized halftone values of luminance are extracted and defined as abnormal patterns. Based on these abnormal patterns, a defect image is generated or their locations on the semiconductor wafer are displayed, thus notifying the operator of the defects. The operator then transmits the data concerning the distribution of the defects displayed as a defect map on the semiconductor wafer to analysis equipment for probing the cause of the defects. In the analysis equipment, several defects are selected from all the defects, and a detailed analysis is conducted using, for example, observation images obtained by the SEM.
As the defect map usually displays many defects with different causes, it is currently up to how experienced the operator is whether he or she can tell which defect is in need of urgent analysis. This is because of the fact that what the defect map displays are merely differences in the images, which are extracted even though they might not actually be defects, and various kinds of defects could be distributed and displayed on the defect map indiscriminately.
To solve this problem, it has been attempted to display not only just the defect map on the inspection apparatus, but to eliminate those defects for which no defect analysis is necessary and then display only the defects of interest and thus output the defect data. For example, an optical inspection apparatus is not capable of detecting non-electrical defects or short defects created during the contact-hole producing process in a semiconductor device formed on a semiconductor wafer. Such defects, however, can be detected by the electron beam inspection apparatus, and attempts have been made to distinguish one type of defect from another based on a defect image and the coordinates of the defect on the semiconductor wafer (see JP Patent Publication No. 2002-124555 A, page 8 and FIG. 6, for example). However, such as the non-electrical and short defects are not necessarily position-dependent, so that it has not always been possible to accurately distinguish these defects based only on the defect map of the semiconductor wafer.
Apart from the semiconductor device manufacturing processes, methods are known for classifying production defects by a plurality of characteristic values such as the profile and luminance of the defects (see JP Patent Publication No. 2002-174603 A, page 3 and FIG. 6, for example). However, no prior art has been found that describes in concrete terms a method of eliminating unnecessary defects from those defects extracted by the electron beam inspection apparatus during the semiconductor device manufacturing process.